A backlight module, also known as a backlight unit, is generally an optical assembly providing a backlight source for products and applied to digital camera, PDA (Personal Digital Assistant), GPS (Global Positioning System), medical apparatus, notebook, LCD Monitor, and etc. Owing to the TFT LCD panel itself does not have a light-emitting function, it must have a light source mounted in the LCD panel. A backlight module is an important optical assembly of a light source. Therefore, a backlight module is a key component for the TFT LCD, a popular product in the present days.
The inner structure of a backlight module basically includes a light source, a reflective sheet, a light guide plate, a diffuser, a prism sheet and a frame base. Technically, a backlight module is firstly made by the method of injection molding that the propylene (acrylic sheet) is pressed to form a sheet with a smooth surface (i.e. a light guide plate). Subsequently, the diffusion spots are printed at the bottom of the light guide plate with high reflective, light-unabsorbable materials by screen printing method. And the light from the cold cathode fluorescent lamp (CCFL) which is situated at the lateral side of the thick end of the light guide plate transmits to the thin end of the light guide plate through reflection. When the light meets the diffusion spots, the reflex scatters by different angles and then transmits from the front side of the light guide plate on account of breaking the total reflection rule. The various sizes of diffusion spots and different spacings between thereof enable the light guide plate to be luminant uniformly. The reflective sheet then reflects the light leaking out from the bottom of the light guide plate back to the light guide plate for increasing the efficiency of light usage.
However, the choke point of developing TFT LCD technology is the brightness. Generally, only 5%˜8% of the light source from the LCD lamp remains after transmitted through substrates to the user's eyes. According to the standard of CRT monitors, the brightness for TFT LCD is not enough. Although plenty of researches are conducted on the solution to improving brightness, such as increasing luminance of backlight source or LCD, i.e. increasing the aperture ratio of TFT LCD or luminance of all components, most of these solutions can not solve the problem without heat dissipating problem and over-consumption of electricity. For the purpose of improving brightness, 3M Company provides a solution by using the brightness enhancement films (BEF) and dual brightness enhancement films (DBEF). A brightness enhancement film is to centralize the scattered light through refracting the lateral light by V-type thin stripes. Using 3M brightness enhancement films may efficiently centralize the light scattered everywhere to the main axis of sight of the display for improving brightness without modifying the structures and over-consumption of electricity. It is a better solution nowadays but the costs are more expensive.
Another solution widely used by industry is to utilize cholesteric liquid crystal (CLC) brightness enhancement film, which is based on the optical rotation property of the cholesteric macromolecules. Please refer to FIG. 1, which is the diagram illustrating the merchandise of CLC brightness enhancement film of Nitto Denko Company. The CLC brightness enhancement film includes the substrate 11, the alignment film 12, and the plurality of CLC layers 13. Each CLC layer 13 has different ranges of transparent wavelength respectively. Through the reflective sheet 151, the light guide plate 152, the ¼ wavelength plate 16, and the polarizer 17 shown in FIG. 1, the light is efficiently transmitted from the light source 14 to the main axis of sight of a display. Since the CLC layers 13 have different optical properties when the cholesteric molecules are cured under different operating conditions, the plurality of CLC layers 13 are made by multiple coating and curing processes. Those processes are very intricate, for example, the CLC layers including red (R), green (G), and blue (B) cholesteric liquid crystal layers, which are three different polarized wavelengths shown in FIG. 1, have to be coated with a cholesteric molecules layer in sequence and subsequently cured under different operating conditions. A further method for manufacturing a plurality of CLC layers is to add dyes into them and manipulate the curing conditions. And the relative processes are more complicated. It is therefore desirous to develop a fast and simple method for manufacturing a plurality of CLC films, i.e. brightness enhancement films.